Inkjet printers print dots on a print medium by ejecting small drops of ink from one or more nozzles. Commonly, a number of nozzles are carried on a moveable print head which can be scanned across a surface of the print medium. Each nozzle may be controlled individually to eject a drop of printing fluid, such as ink when commanded. By appropriate control of each nozzle as the print head carrying the nozzles is scanned across the surface of the print media, a desired pattern of printing fluid drops can be deposited on the print medium.
In order to print a color image using a limited number of colors of ink, halftone techniques are used whereby each pixel of the color image is formed from a number of dots of ink of the available colors. For each color of ink deposited a limited number of drops of ink may be deposited for each pixel in a single pass. Typically, this is limited to a single drop of ink of a particular color being deposited at a given pixel in a single pass as the print head is scanned across the print medium.
Thus, while light color pixels may be printed in a single pass of the print head, darker color pixels may require more ink drops to be deposited and, therefore, require multiple passes of the print head.
Print heads may carry an array of nozzles allowing ink to be deposited in a number of lines of pixels, or a swath, in a single pass of the print head. To allow a full image, consisting of a number of swathes to be deposited on the print medium, the medium is advanced through the printer to allow subsequent swathes to be deposited. However, it is common for errors in the advance step to result in undesired artifacts in the final image produced.
Typically, if the print medium is advanced by too small amount, printed swathes may overlap leading to darker or more saturated bands of color, while advancing the print medium by too large a step may result in areas between swathes with insufficient or no ink drops. Such banding is undesirable.